NCEES 2008 # 516 Thermo and Fluids ?

[tmacier]

I am working thru the problems that I dont understand and this is one of them.

I fully understand the approach (working it matching 27-10 of the MERV)

The answer says that at 96 psia h2'=1,201 btu/lbm

I worked this using the quaility approach and recogizing that s1=s2 -

THe propblem I see is when I look at the saturated steam table and try to find hf+(x)hfg the answer they provide is above the hg value?!

In other words, using table 24.b the 1,201 btu/lbm shown is the answer cannot be higher than the 1186 shown on the steam table?

Can someone take the time and tell me what I am missing?

 

[NerdHerd]

I am working thru the problems that I dont understand and this is one of them.
I fully understand the approach (working it matching 27-10 of the MERV)

The answer says that at 96 psia h2'=1,201 btu/lbm

I worked this using the quaility approach and recogizing that s1=s2 -

THe propblem I see is when I look at the saturated steam table and try to find hf+(x)hfg the answer they provide is above the hg value?!

In other words, using table 24.b the 1,201 btu/lbm shown is the answer cannot be higher than the 1186 shown on the steam table?

Can someone take the time and tell me what I am missing?

_____________

Hello Tim,

For this problem, State 1 & State 2 occur in the super heated region, therefore the equations of hf+(x)hfg are not applicable, because the states do not fall within the dome. Just as you already noticed, hg = the value of saturated vapor. The SH region is located to the right of hg, which is beyond the limits of the dome. Therefore, you must interpolate once again for h2. Once you interpolate, you know that S1=S2, and there is no quality or sf or sg to use at this point, therefore you must turn to the Mollier Diagram located on A-53 of MERM.

Again, all of this is not mentioned in the book, which really annoys me, as I’m sure it does you.

I hope this helps. Good luck.

NerdHerd

 

[tmacier]

It has been awhile since I have been in school and I am a bit embarresed to state that I forgot about Mollier!

Just relearned it and the material makes perfect sense.

Thanks again for the help!

I cant believe how much I have forgoten since schooling!

Tim

 

[abourne]

I spent a lot of time on this problem as well.

Thank god I have a large 11 x 17 Mollier diagram with colorful lines.

Even with that, I had so much trouble keeping the 900 psia and 900-F point, 1.6xxx something entropy, and using my triangle to go isentropically down to the 96 psia line (estimated 100)

I estimated about 1200 BTU/lbm.

What sucks is that if you're off by 20 or so BTU/lbm, you'll get the problem wrong, so BE CAREFUL and PRECISE using the Mollier.

Also, don't forget efficiency is W_Actual / W_ideal for the turbine (or as I like to think of it, small / big, b/c it's always less than 1.0)

The solution put the "prime" in the numerator using a different nomenclature than MERM, but it's correct, because they have Actual / Ideal.